This revised proposal is aimed at determining the role of neurotrophins that signal through Trk receptors in modulating synapse structure and function in the developing and adult nervous system. The mouse neuromuscular junction will be used as a model system. The cell types that comprise neuromuscular junctions, the perisynaptic Schwann cells, presynaptic motor neuron terminals, and postsynaptic muscle fibers, each express a complement of neurotrophins and Trks, suggesting that neurotrophin signaling at this synapse is multi&#8209;directional and involves all three cell types. Similarly, CNS neurons express several neurotrophins and Trks, but the relative roles of each signaling pathway in synaptic maturation and maintenance are unclear. Recent work from our lab showed that TrkB isoforms (which bind the neurotrophins BDNF and NT4/5) is expressed primarily postsynaptically, in the muscle fiber membrane in and around acetylcholine receptor (AChR) rich regions, while TrkC isoforms are localized to perisynaptic Schwann cells and TrkA is not localized to neuromuscular junctions. Down-regulation of TrkB signaling in muscle fibers, via adenovirus-mediated over-expression of a truncated, non-signaling form of TrkB (TrkB.tl), induced the dismantling of postsynaptic AChR rich regions. These observations lead to the hypothesis that exchange of ligands that signal through TrkB or TrkC receptors play different roles in synaptic maturation and maintenance. To test this hypothesis, we will use adenovirus&#8209;mediated manipulation of neurotrophin and Trk expression in different cell types at neuromuscular junctions in vivo. The effect of these manipulations on neuromuscular junction structure and function will be analyzed with in vivo imaging, immunostaining, confocal microscopy and electrophysiological characterization of synaptic strength. We will also explore the molecular mechanisms downstream of TrkB&#8209;mediated signaling, and how these may interact with the agrin/MuSK signaling pathway that mediates AChR clustering. The results of these experiments will provide new insights into the functional role(s) of neurotrophin and Trk&#8209;mediated signaling at developing and adult synapses, and extend our understanding of the relative roles of antero&#8209; and retrograde synaptic signaling in the peripheral as well as central nervous system.